Advertisements (or ads) can be placed at various locations on a web page. Advertisements can also be of different size or shape, or have different content requirements. An advertisement suitable for placement at one location of a web page may not be suitable at another location. In addition, advertisements placed at different locations of the web page perform differently. Furthermore, web page content and layout can be changed, for example by web page publishers, from time to time. Such changes can alter the location of advertisements on the web page.